1. Industrial Applicability
The present invention finds applicability in the transportation industries, and more specifically private and commercial vehicles. Of particular importance is the invention""s incorporation into hybrid electric vehicles.
2. Background Art
Generally, a hybrid electric vehicle combines electric propulsion with traditional internal combustion engine propulsion to achieve enhanced fuel economy and/or lower exhaust emissions. Electric propulsion has typically been generated through the use of batteries and electric motors. Such an electric propulsion system provides the desirable characteristics of high torque at low speeds, high efficiency, and the opportunity to regeneratively capture otherwise lost braking energy. Propulsion from an internal combustion engine provides high energy density, and enjoys an existing infrastructure and lower costs due to economies of scale. By combining the two propulsive systems with a proper control strategy, the result is a reduction in the use of each device in its less efficient range. Furthermore, and as shown in FIG. 1 regarding a parallel hybrid configuration, the combination of a downsized engine with an electric propulsion system into a minimal hybrid electric vehicle results in a better utilization of the engine, which improves fuel consumption. Furthermore, the electric motor and battery can compensate for reduction in the engine size.
In typical configurations, the combination of the two types of propulsion systems (internal combustion and electric) is usually characterized as either series or parallel hybrid systems. In a pure series hybrid propulsion system, only the electric motor(s) are in direct connection with the drive train and the engine is used to generate electricity which is fed to the electric motor(s). The advantage of this type of system is that the engine can be controlled independently of driving conditions and can therefore be consistently run in its optimum efficiency and low emission ranges. A key disadvantage to the series arrangement is the loss in energy experienced because of the inefficiencies associated with full conversion of the engine output to electricity.
In a pure parallel hybrid propulsion system, both the engine and the electric motor(s) are directly connected to the drive train and either one may independently drive the vehicle. Because there is a direct mechanical connection between the engine and the drive train in a parallel hybrid propulsion system, less energy is lost through conversion to electricity compared to a series hybrid propulsion system. The operating point for the engine, however, can not always be chosen with full freedom.
The two hybrid propulsion systems can be combined into either a switching hybrid propulsion system or a power-split hybrid propulsion system. A switching hybrid propulsion system typically includes an engine, a generator, a motor and a clutch. The engine is typically connected to the generator. The generator is connected through a clutch to the drive train. The motor is connected to the drive train between the clutch and the drive train. The clutch can be operated to allow series or parallel hybrid propulsion.
A power-split hybrid system, as is exemplarily employed with respect to the present invention, includes an engine, a generator and a motor. The engine output is xe2x80x9csplitxe2x80x9d by a planetary gear set into a series path from the engine to the generator and a parallel path from the engine directly to the power train. In a power-split hybrid system, the engine speed can be controlled by varying the power split to the generator by way of the series path, while maintaining the mechanical connection between the engine and drive train through the parallel path. The motor augments the engine on the parallel path in a similar manner as a traction motor in a pure parallel hybrid propulsion system, and provides an opportunity to use energy directly through the series path, thereby reducing the losses associated with converting the electrical energy into, and out of chemical energy at the battery.
In a typical power-split hybrid system, the generator is usually connected to the sun gear of the planetary gear set. The engine is connected to the planetary carrier and the output gears (usually including an output shaft and gears for interconnection with the motor and the wheel-powering, final drive train) are connected to the ring gear. In such a configuration, the power-split hybrid system can generally be operated in four different modes; one electric mode and three hybrid modes.
In the electric mode, the power-split hybrid system propels the vehicle utilizing only stored electrical energy and the engine is turned off. The tractive torque is supplied from the motor, the generator, or a combination of both. This is the preferred mode when the desired power is low enough that it can be produced more efficiently by the electrical system than by the engine and when the battery is sufficiently charged. This is also a preferred mode for reverse driving because the engine cannot provide reverse torque to the power train in this configuration.
In the parallel hybrid mode, the engine is operating and the generator is locked. By doing this, a fixed relationship between the speed of the engine and the vehicle speed is established. The motor operates as either a motor to provide tractive torque to supplement the engine""s power, or can be operated to produce electricity as a generator. This is a preferred mode whenever the required power demand requires engine operation and the required driving power is approximately equal to an optimized operating condition of the engine. This mode is especially suitable for cruising speeds exclusively maintainable by the small internal combustion engine fitted to the hybrid electric vehicle.
In a positive split hybrid mode, the engine is on and its power is split between a direct mechanical path to the drive train and an electrical path through the generator. The engine speed in this mode is typically higher than the engine speed in the parallel mode, thus deriving higher engine power. The electrical energy produced by the generator can flow to the battery for storage or to the motor for immediate utilization. In the positive split mode, the motor can be operated as either a motor to provide tractive torque to supplement the engine""s power or to produce electricity supplementally with the generator. This is the preferred mode whenever high engine power is required for tractive powering of the vehicle, such as when high magnitude acceleration is called for, as in passing or uphill ascents. This is also a preferred mode when the battery is charging.
In a negative split hybrid mode, the engine is in operation and the generator is being used as a motor against the engine to reduce its speed. Consequently, engine speed, and therefore engine power, are lower than in parallel mode. If needed, the motor can also be operated to provide tractive torque to the drive train or to generate electricity therefrom. This mode is typically never preferred due to increased losses at the generator and planetary gear system, but will be utilized when engine power is required to be decreased below that which would otherwise be produced in parallel mode. This situation will typically be brought about because the battery is in a well charged condition and/or there is low tractive power demand. In this regard, whether operating as a generator or motor, the toque output of the generator is always of the same sense (+/xe2x88x92); that is, having a torque that is always directionally opposed to that of the engine. The sign of the speed of the generator, however, alternates between negative and positive values depending upon the direction of rotation of its rotary shaft, which corresponds with generator vs. motor modes. Because power is dependent upon the sense of the speed (torque remains of the same sense), the power will be considered to be positive when the generator is acting as a generator and negative when the generator is acting as a motor.
When desiring to slow the speed of the engine, the current being supplied to the generator is changed causing the speed of the generator to slow. Through the planetary gear set, this in turn slows the engine. This effect is accomplished because the resistive force acting against the torque of the generator is less at the engine than at the drive shaft which is connected to the wheels and is being influenced by the entire mass of the vehicle. It should be appreciated that the change in speed of the generator is not equal, but instead proportional to that of the engine because of gearing ratios involved within the connection therebetween.
In electric and hybrid electric vehicles, large capacity electricity storage device(s), usually in the form of battery packs, are required. By conventional design, these batteries include a plurality of cylindrical battery cells that are collectively utilized to obtain sufficient performance and range in the vehicle. Typically, batteries are positioned within the vehicle in a compartment configured to protect against damage and to prevent the cells, and mostly their acidic contents, from causing injury or damage, especially in the event of a crash. When stored in these typically confined compartment(s), heat buildup generated from use and/or charging affects the endurance of the batteries, and in some circumstances can destroy individual battery cells. Traditional cooling of the batteries and the battery compartment requires increasing the volume of the compartment for air cooling and/or running cooling hoses to external radiators.
Typically, to achieve a smooth engine start in a hybrid electric vehicle in which the engine is mechanically interconnected with the drive wheels, the start of engine fuel injection and ignition are made at revolutionary speeds above any mechanical resonance speeds of the drive train. Additionally, at full take-off acceleration, any delay in the engine""s production of power typically decreases engine performance. Still further, to achieve smooth driving characteristics and obtain low fuel consumption, the engine torque and speed change rates must be limited. At full take-off, this usually results in an increased time period for the engine to reach maximum power, and all of these conditions deteriorate acceleration performance of the vehicle.
As can be appreciated, the engine is not always running during vehicle operation. If the engine is stopped for a sufficiently long period during the operation of the vehicle, the exhaust system catalyst may cool down too much, and to such a degree that a temporary, but significant increase in exhaust emissions occur upon restart and until the catalyst once again warms to its effective temperature.
In another aspect, the battery state-of-charge (SOC) in a hybrid electric vehicle is typically controlled using SOC feedback control. When applying SOC feedback control, however, and when the vehicle is operating in a low velocity region, the SOC feedback control tends to grow unstable as velocity increases. Instability also occurs when the vehicle is operating at high velocity and the velocity of the vehicle then decreases. The same instability or weakness can still occur even when using xe2x80x9cfeed-forwardxe2x80x9d type estimating of required tractive force; the same being a typical complement to SOC feedback control. This is particularly true at low vehicle velocities with velocity increases and at high vehicle velocities with velocity decreases. Even when properly designed, the SOC feedback control can also be weak at full take-off.
In a typical power-split hybrid electric propulsion arrangement, the control strategy advantageously involves operating the engine along optimum efficiency torque vs. speed curves. A trade-off exists between traction force performance and fuel economy which, for optimization, typically requires selection of a particular gear ratio between the engine and the wheels that causes the engine to deliver more power than is needed for vehicle propulsion. This generally occurs at cruising in parallel mode, or near constant vehicle velocity conditions. Operation under these conditions can, sometimes, cause the battery and charging system to reject energy being presented thereto from the engine. This problem is generally solved by decreasing or limiting the engine output power by entering negative split mode which entails using the generator as a motor to control the engine to a decreased speed. Such control allows the engine to follow an optimum curve at reduced engine output power.
Use of the generator as a motor gives rise to a power circulation in the power-train which leads to undesirable energy losses at the generator, motor, inverters and/or planetary gear set. These energy losses may be manifest as heat generation which indicates that most efficient use is not being made of the installed drive train.
In a power-split hybrid propulsion system having planetary gear set(s) and utilizing a generator lock-up device, a harshness in ride occurs when the generator lock-up device is engaged or released. This is due primarily to the difference in how engine torque is estimated when the vehicle is in different operating modes. Typically, when the generator is locked up, engine torque is estimated from the combustion control process of the engine. When the generator is free, as in split mode, however, engine torque is estimated from the generator torque control process. The difference in values of these two estimating techniques gives rise to what usually amounts to a variation in operating torque between the engine and generator when the lock-up device is engaged or disengaged, thereby creating harshness in the vehicle""s operation, usually manifest as abrupt changes or jerkiness in the vehicle""s ride.
As earlier indicated, the generator is typically used to control the engine in power-split hybrid mode. This is usually accomplished by employing a generator having maximum torque capabilities substantially greater than the engine""s maximum torque that is transmittable to the planetary gear system. Failure to have such a control margin can result in generator over-speed and possible damage to the propulsion system. Such a control margin means, however, that the engine and generator are not fully exploited at full capacity acceleration.
Several deficiencies associated with the use of known hybrid electric vehicle designs and methods of operating the same have been described hereinabove. Several inventive arrangements and methods for operating hybrid electric vehicles are described hereinbelow that minimize, or remedy these deficient aspects of known designs, and/or provide benefits, in and of themselves, to the user. These new, improved and otherwise potentiated solutions are described in greater detail hereinbelow with respect to several alternative embodiments of the present invention.
In a first aspect, an arrangement for a compact battery and cooling system therefore is disclosed. The arrangement includes a plurality of elongate battery cells, each battery cell having a longitudinal axis and a hexagonal cross-sectional shape in a plane oriented substantially perpendicular to the longitudinal axis. The cells are parallelly oriented, each to the others, within a battery housing. Preferably, the cells are arranged in a honeycomb configuration with opposed faces of adjacent battery cells proximately located one to the other in face-to-face relationship. At least one substantially hexagonally shaped cooling channel is provided at an interior location within the plurality of battery cells.
In a second aspect, a method for potentiating an engine""s power contribution to a hybrid electric vehicle""s performance in a take-off operating condition is disclosed. Normally, fuel injection to, and ignition at the engine are only commenced when the engine is operating at a speed exceeding the resonance speed of the drive train to reduce engine start harshness; such resonance speeds of the drive train being dictated, at least in part, by transmission backlash, softness and the like. During high driver acceleration demands, however, ignition and the injection of fuel is desirably started as early as possible to potentiate output power and acceleration.
In a third aspect, a method for maintaining a catalyst of an emissions system in a hybrid electric vehicle in an operative condition is disclosed. The method includes sensing that an engine of a hybrid electric vehicle has stopped operating. A time period is predicted after which a catalyst of an emissions system associated with the engine will cool to a light-off temperature below which the catalyst becomes ineffective. The predicting step is based on known qualities of the catalyst and ambient conditions in which the vehicle is being operated. The engine is restarted when the predicted time period has expired thereby maintaining the catalyst at temperatures in excess of the light-off temperature.
In a fourth aspect, a method for minimizing driver perceptible drive train disturbances during take-off in a hybrid electric vehicle when maximized power is often desired is disclosed. The method includes sensing an actual state-of-charge (SOC) value of a battery in a hybrid electric vehicle and a traveling velocity of the vehicle during take-off operation. The sensed actual SOC value is compared with a SOC reference value and computing a delta SOC value as a difference therebetween. A velocity-based SOC calibration factor is looked up that corresponds to the traveling velocity of the vehicle. A combination is utilized of the delta SOC value and the SOC calibration factor as a SOC feedback engine speed control instruction to an engine controller of the hybrid electric vehicle. A driver""s desired vehicular acceleration is sensed based on accelerator position. Maximum possible engine power generatable at the sensed vehicle speed is determined, as is a required power value from the power train of the vehicle to meet the driver""s desired vehicular acceleration. The maximum possible engine power generatable at the sensed vehicle speed is compared with the required power value and computing a delta power train requirement value as a difference therebetween. A velocity-based and accelerator position-based power calibration factor is looked-up that corresponds to the traveling velocity of the vehicle and the accelerator position. A combination of the delta power train requirement value and the power calibration factor is utilized as a power requirement feed-forward engine speed control instruction to an engine controller of the hybrid electric vehicle.
In a fifth aspect, a method for optimizing the operational efficiency of a hybrid electric vehicle is disclosed. The method comprises operating an engine of a hybrid electric vehicle preferentially on an optimized power curve of the engine for maximizing the efficiency of the engine and sensing a state-of-charge (SOC) condition of a battery of the hybrid electric vehicle being at a preferential value indicative of no additional charging being desired. The running torque of the engine is reduced below the optimized torque curve to a point that the power produced by the engine is substantially equal to the power demanded for driving the hybrid electric vehicle.
In a sixth aspect, a method for calibrating and synchronizing sensed operating torques of an engine and a generator in a planetary gear based hybrid electric vehicle is disclosed. The method includes providing a sensor that detects the operational torque of an engine of a hybrid electric vehicle at the engine""s interface with a planetary gear system of the hybrid electric vehicle. A sensor is provided that detects the operational torque of a generator of a hybrid electric vehicle at the motor""s interface with the planetary gear system of the hybrid electric vehicle. The planetary gear system of the hybrid electric vehicle is operated in a split mode so that the generator is directly linked to the engine and a reading of the sensor that detects the operational torque of the generator may be used to compute the operating torque of the engine. Paired values of sensed operational torques of the engine and the generator at like times are recorded. Each pair of recorded values are arithmetically processed and calibrating value is computed therebetween. The sensing and recording of paired values is repeated at the same sensed generator and engine speeds and torques thereby enabling the calculation of computed average calibrating values at each of the particular sensed generator speeds suitable for subsequent utilization in computing corresponding engine torques in the future. The engine and the generator are controlled utilizing the average calibrating value at future times of transition between power-split mode and parallel mode of the planetary gear system so that the engine is substantially synchronized with the generator at the time of direct linkage across the planetary gear arrangement thereby avoiding driver detectible irregularities in the performance of the power train of the hybrid electric vehicle.
In a seventh aspect, a method for potentiating the utilizable torque output capacity of a hybrid electric vehicle is disclosed. The method includes controlling operation of an engine of a hybrid electric vehicle using a generator, the engine and generator being interconnected through a planetary gear system, the generator having approximately equal torque output capacity as the engine based on connective gear ratio selection. An engine controller is utilized for managing the engine""s operation thereby permitting the engine to be operated at a torque output level substantially equal to the maximum torque output of the generator without a significant margin of excess control capacity of the generator over the engine. An overpower condition is detected in which the torque output of the engine is surpassing the maximum torque output of the generator. The engine is controlled to a torque output that is less than the maximum torque output of the generator.
The general beneficial effects described above apply generally to the exemplary descriptions and characterizations of the devices, mechanisms and methods disclosed herein. The specific structures and steps through which these benefits are delivered will be described in detail hereinbelow.